1. Field of the Disclosure
This disclosure relates generally to control of a rotating electric machine, and more specifically, to a power factor-based speed regulation device and method for an inverter fed induction motor drive system.
2. Description of Related Art
Many applications for electric motors require variable speed motor operation, and to this end, various speed control solutions have been proposed. Induction motors are popular for several reasons, including high robustness, reliability, low price and high efficiency. A typical induction motor includes a stationary member, or stator, that has a plurality of windings disposed therein. A rotating member, or rotor, is situated within the stator to rotate relative thereto. In a three-phase induction motor, for example, a rotating magnetic field is established by applying three-phase sinusoidal alternating voltages to the stator windings. The rotating magnetic field interacts with the rotor windings to effect rotation of the rotor.
Power conversion systems are commonly used to provide the multiphase AC power to the induction motor for variable speed applications. An example of such a power conversion system is a DC-to-AC inverter bridge, which typically includes power semiconductor switching devices connected in a bridge formation between the DC bus lines and output terminals of the power conversion system. The switching devices are controlled to connect the power on the DC bus lines to the system output terminals in a desired pattern such that AC output signals having the desired fundamental frequency and amplitude are synthesized from the DC power on the DC bus lines. Various modulation strategies may be employed for controlling the inverter switching devices to deliver power, including sine wave Pulse-Width Modulation (xe2x80x9cPWMxe2x80x9d).
Clothes washing machines often employ induction motors. The need to maintain washing machine drum speeds within required specifications has typically required the use of a tachometer on the motor shaft to provide a speed feedback to the motor controller. The desired speeds are achieved by controlling the excitation frequency and the corresponding voltage. Low cost speed control solutions are often implemented using constant or schedule based volts-per-hertz algorithms. Speed regulation is based on feedback from the tachometer attached to the rotor. Elimination of the tachometer, however, is desirable not only from a cost perspective, but also for reliability reasons.
The natural characteristic of the induction motor will allow the rotor speed to decrease with increasing torque load on the shaft, at constant voltage amplitude and frequency. To counter this and maintain a more constant speed, speed control methods vary the voltage and frequency to control the speed of the rotor. A secondary purpose of this is to prevent saturation of the motor stack, which will lead to over heating of the motor. Thus, control schemes used in applications where the load on the motor shaft varies over a wide range (for example, a washing machine) should be capable of applying proper stator voltage amplitude and frequency to the motor so as to maintain shaft speed and prevent over heating of the motor. Moreover, it is desirable to control the motor over a wide range of speeds.
Several solutions have been proposed for efficient operation of an induction motor based on controlling the power factor of the motor (generally, the power factor is calculated based on the phase difference between the voltage and currents). Such solutions, however, do not adequately address the requirements of a variable speed application such as that mentioned above. They are primarily designed to provide efficient operation of the motor by optimizing the power factor of the motor.
The present invention addresses shortcomings associated with the prior art.
In accordance with aspects of the present invention, a control method and system for an induction motor is disclosed. The motor includes a rotor and a stator with a plurality of phase windings therein to which AC power is applied to cause rotation of the rotor relative to the stator. The control method may be stored as program instructions on a machine-readable medium and implemented by a digital controller such as a digital signal processor (DSP) chip, microcontroller or microprocessor. The control method includes receiving a rotor speed command signal and receiving an indication of the phase winding current. The motor power factor is estimated based on the current waveform, and the rotor speed is estimated based on the estimated power factor. The estimated rotor speed and the rotor speed command signal are compared to generate a speed error, and in response the speed error, a voltage signal applied to the phase windings is adjusted.
More specifically, in exemplary embodiments, the controller estimates the motor power and power factor based on the voltage input to the phase windings and the current in the phase windings. The slip speed of the motor is estimated based on the calculated power factor, calculated motor power, voltage applied, and frequency of the applied voltage. The estimated slip speed is used to calculate the rotor speed. The rotor speed is compared with the commanded rotor speed to generate a speed error. Based on this error, the voltage amplitude and frequency are adjusted to drive the speed error to zero. The estimate of rotor speed may be compensated in response to the motor temperature variation.